Composite reinforcement cord

ABSTRACT

A composite cord for reinforcing a tire or other elastomeric article having at least one high strength, substantially inextensible yarn spirally wrapped around a core. The core at room temperature has sufficient strength and elasticity to resist and recover from tension forces on the core required during the processing of the cord and building of the tire. The core is composed of material which loses its tensile strength when subjected to temperatures of vulcanization to permit stretching out of the yarn to allow for substantial elongation of a part or all of the tire and then provide reinforcement of the elongated tire by the stretched out, high strength yarn.

BACKGROUND OF THE INVENTION

The invention is directed to a composite reinforcement tire cord similarto cords having a relatively weak core around which much stronger,inextensible yarns or cords are coiled, such that the composite cordhas, initially, a low tensile modulus which abruptly changes to asubstantially higher modulus upon predetermined elongation of the cord.The cores of the tire cords used heretofore are typically composed ofeither a low tensile material such as a single strand of cotton, rayon,nylon or polyester, which is readily breakable, or a vulcanizedelastomeric material which is stretchable and allows the inextensibleyarns to uncoil and straighten out during the toroidal shaping andmolding of the tire.

The invention is directed to the provision of a composite, highlyelongatable cord having a core which, at room temperature under tension,has a barrier to elongation and elastic recovery. Further, the core iscomposed of material that loses its tensile strength when subjected tohigh temperature during the molding and vulcanization operations so asto permit elongation of the yarns wrapped around the core.

Briefly stated, the invention is in a composite cord for reinforcingelastomeric articles such as tires. The composite cord comprises aplurality of substantially inextensible yarns which are spirally wrappedaround a core which has an elastic limit, at low elongation, sufficientto maintain the configuration of the cord during processing, but whichloses its tensile strength during the molding and vulcanization of theelastomeric article to permit extension of the cord after beingsubjected to the temperature of vulcanization.

DESCRIPTION OF THE DRAWING

The following description of the invention will be better understood byhaving reference to the annexed drawing, wherein:

FIG. 1 is a section of an inflated tire mounted on a wheel rim,illustrating certain uses of the composite cord of the invention and thetire being shown in the deflated condition in a chain-dotted line.

FIG. 2 is a view of a composite cord made in accordance with theinvention;

FIG. 3 is a diagram illustrating the thermoplastic behavior of the coreof the composite cord of FIG. 2;

FIG. 4 is a stress-strain diagram of the composite cord of FIG. 2, priorto subjecting the cord to temperatures above 120° C.; and

FIG. 5 is a stress-strain diagram of the composite cord of FIG. 2, afterthe cord has been subjected to temperatures up to 156° C.

DETAILED DESCRIPTION

Referring more particularly to FIG. 1, there is shown a tire 5 which ismounted and inflated on a wheel rim 6. The tire 5 comprises a fluidimpervious innerliner 7; at least one carcass ply 8, including a layerof reinforcement cords 9, adjacent the innerliner 7; and a thread 10 andpair of sidewalls 11 and 12 which surround the carcass ply 8 andterminate at a pair of annular metal beads 13 and 14. A belt structure15, comprising a pair of superimposed belt plies 16 and 17 is positionedbetween the tread 10 and carcass ply 8 to reinforce the tire 5. Acarcass overlay, or belt underlay 18, is interposed between the beltstructure 15 and carcass ply 8. The tire 5 is preferably of the radialtype wherein the reinforcement cords 9 of the carcass ply 8 are disposedat angles of from 75 to 90 degrees measured in relation to a planecontaining the mid-circumferential centerline of the tread 10, suchplane hereafter referred to as the centerplane. The belt plies 16 and 17are reinforced with cords disposed at angles in the range of from 16 to24 degrees relative to the centerplane. The cords of the belt plies 16and 17 cross the centerplane in opposite directions. The cord angles ofthe belt plies 16 and 17 are the same, but lie in opposite directionsfrom the centerplane. The cord angles are in relation to a tire which ismolded, vulcanized and uninflated. The tire 5 may also be of a radialtype designed to be molded with a smaller diameter than the inflatedtire, shown in a chain-dotted line adjacent the wheel rim 6.

Referring now to FIG. 2, there is shown a composite cord 20 which isutilized to reinforce various components of the tire 5, e.g. the carcassply 8, the belt structure 15, or the carcass overlay 18. The compositecord 20 comprises a core 21 with a spiral or helical wrapping of one ormore substantially inextensible yarns 22, 23.

The core 21 is of a low molecular weight polymeric material selectedfrom the group comprising polyolefines including polyethylene andpolypropylene. A particularly good core 21 was found to be a polymericmonofilament of polyethylene or polypropylene having a density in therange of from 0.9 to 1.0 grams per cubic centimeter (gr/cm³). The core21 at room temperature may have an elastic limit of at least 2.5kilograms and preferably of at least 1.5 grams per denier (gr/dn), abreaking strength less than 20 kilograms, and an initial modulus ofelasticity of at least 25 grams per denier in the load range of from 0to 2.5 kilograms.

The core 21 is degradable at temperatures normally used to vulcanizerubber which are usually over 120° C. and in the broad range of from120° C. to 150° C., and in the preferred narrower range of from 130° C.to 140° C. After the core 21 is subjected to these elevatedtemperatures, the modulus of elasticity is reduced to 4 grams per denieror less and remains at the reduced amount when it is cooled to lowertemperatures such as the outside temperatures at which a tire is used.The tenacity of the core 21 is also reduced to zero or a negligibleamount of around 1 gram per denier after the core is subjected to thesetemperatures. It is not entirely understood but it is believed the lossin tensile strength of the core 21 results from a partial or total lossof crystalline structure to induce molecular slips upon the applicationof stress.

The core 21 described above consists of one filament; however, in someapplications it may be desirable to have more than one filament, forexample, three. The core 21 may be coated with a rubberized compoundbefore the yarns 22,23 are wound around the core. It may also bedesirable to use a stapled polymer monofilament similar to cottonfilaments which is completely coated with a rubberized compound beforeit is incorporated into the cord 20 to provide improved adhesion withthe yarns 22,23 of the cord.

The yarns 22,23 which are spirally wrapped in the form of a helix aroundthe core 21, may be composed of any suitable reinforcing material whichis used for conventional tire reinforcement cords having a tenacity ofat least 6 grams per denier. Among these materials are rayon, nylon,polyester, aramid and steel. Each of the yarns 22,23 is formed of atleast one strand that is composed of filaments which are twisted aroundeach other. Yarns 22,23, composed of one strand having a denier in therange of from 800 to 1500 and helically wrapped around the core 21 ineither an S- or Z-direction at a rate of from 3 to 7 turns percentimeter, were successfully used to form a good composite cord 20which was elongatable between 50 percent and 100 percent. The yarns22,23 may also be composed of a plurality of strands which are cabledtogether, generally in a direction which is opposite that which thefilaments of each strand are twisted.

EXAMPLE OF A COMPOSITE CORD

A good composite core 20 comprises a core 21 which is a monofilamentthat is composed of polyethylene having a density of 0.96 gr/cm³ at roomtemperature of about 20°-22° C.

Two substantially inextensible yarns 22,23 are spirally wrapped aroundthe center core 21 in opposed lay. The yarns 22,23 are each formed of asingle, 1500 denier strand which is composed of filaments of aramidtwisted together. The twist in the individual strands or yarns 22,23, inthis case, is in a Z-direction at a rate of 2.5 turns per centimeter.The twist of the yarns 22,23 around the core 21 is in an S-direction ata rate of 6 ns per centimeter. Due to the mutually opposed lay of theyarns 22,23 around the core 21, the yarns 22,23 will, during elongationof the composite cord 20 after subjecting the core 21 to a temperatureused for vulcanization, twist into a single twisted cord of the typeconventionally used in the reinforcement of tires.

The polyethylene monofilament core 21 of this example has the followingproperties at room temperature:

    ______________________________________                                        density                 0.96 gr/cm.sup.3                                      diameter                0.50 mm                                               breakihg strength       10 kg                                                 elastic limit           9 kg                                                  Young modulus at 2.5 1 g                                                                              75 gr/dn                                              elongation at elastic limit                                                                           17%                                                   elastic recovery at 3% elongation                                                                     90%                                                   denier                  1700                                                  ______________________________________                                    

Referring to FIG. 3, the thermoplastic behavior of the polyethylenemonofilament core 21 is charted as a function of the breaking strengthin kilograms vs. the temperature in degrees centigrade. Morespecifically, FIG. 3 shows the breaking strength of the core 21, after 5minutes of heat treatment at the respective temperatures. As can beappreciated from FIG. 3, the breaking strength of the polyethylene core21 remains unaffected by the temperature up to about 122° C. at whichtemperature the breaking strength starts to decrease. By processing athigher temperatures the breaking strength of the core 21 is furtherreduced and at about 142° C. the breaking strength is reduced tosubstantially zero. Thus, it will be appreciated that the polyethylenemonofilament core 21 will not lose its original tensile resistanceduring processing of the cord 20, if it is not subjected to temperaturesabove 120° C. On the other hand, it will be appreciated that the core21, when embedded in a tire 5, will, unlike cores of prior art cords,lose its tensile resistance during vulcanization of the tire which isnormally carried out at about 150° C.

Referring now to the stress-strain diagram of FIG. 4, the cord 20 withthe polyethylene monofilament core 21 passes through three phases orstages. First, there is an intial high modulus phase A-B which the cord20 experiences as the core 21 stretches to the point where it loses itstensile resistance. Secondly, the cord 20 passes through a low modulusphase C-D where the inextensible yarns 22,23 are stretched out and thetensile strength of the cord gradually increases. Thirdly, the cord 20passes through the final, high modulus phase D-E where the uncoiledyarns 22,23 are twisted and tensioned until they break at point E.

It will be appreciated that the initial region A-B forms a barrier toappreciable elongation of the cord 20 for tensiles up to around 12kilograms. Within this barrier, the cord 20 has a high modulus and highelastic recovery. Thus, in consequence of this barrier, the cord 20 hassufficient strength to resist and recover from any tensional forcesimparted to the cord during its processing or the processing of tirecomponents reinforced with such cords, before the shaping andvulcanization of a tire composed of such components.

A typical processing of the cord 20 includes mechanical and thermalstresses, the latter being experienced by the cord as it is exposed totemperatures of from 100° C. to 120° C. to dry the cord, after it hasbeen coated with a resorcinol formaldehyde type adhesive for promotingthe bond between the cord and rubbery material used in the production oftires.

Referring now to the stress-strain diagram of FIG. 5, the same cord 20with the polyethylene monofilament core 21 has lost its tensileresistance after it has been subjected to 15 minutes of heat treatmentat 156° C. The elongation barrier A-B of the cord 20, when unheated, asseen in FIG. 4, substantially disappears. There is only a low modulussubstantially linear region A'-D' corresponding to the high elongationof the cord 20 as the yarns 22,23 stretch out, and then a high modulusphase D'-E' corresponding to the extension of the twisted yarns.

Thus, it can be appreciated that the composite tire cord 20 of theinvention provides a substantial advantage over prior art compositecords in that it has an initial, elastic, high modulus elongationbarrier providing sufficient resistance and recovery of the cord withrespect to any processing tensions experienced by the cord beforeshaping and vulcanization of the tire, whereas after vulcanization thecord has no such barrier and may be elongated with a minimum of effort.

The composite cord 20 of the invention is particularly well suited foruse in a radial spare tire, as described in copending U.S. applicationSer. No. 916,270 now abandoned which was concurrently filed with thisapplication. The copending application covers a radial tire havingcomposite cords in the carcass and the belt. The tire is vulcanized inthe radially reduced configuration where the composite cords are stillin a helical condition. As shown in FIG. 1, a radial tire 5 may havesuch composite cords 20 in carcass plies 9, in belts 16,17 of beltstructure 15 and in a carcass overlay 18, and can be molded andvulcanized in a shape (shown by the chain-dotted line) having a reduceddiameter while the yarns 22,23 are still in a helical condition.

It should be apparent from the above description that duringvulcanization, the core 21 degrades and the elongation barrier A-B ofFIG. 4 disappears. Therefore, after vulcanization, the tire 5 is easilyexpanded to its normally inflated and running condition as the yarns22,23 are readily elongated and twist into twisted reinforcing cords.The composite cord 20 can also be utilized in the carcass overlay 18described in copending U.S. application Ser. No. 916,272 now U.S. Pat.No. 4,240,486 which was concurrently filed with this application. Thecopending application is directed to a tire in which the carcass ply 8has conventional cords and where it is desirable to have the cords inthe overlay 18 at very low cord angles relative to the centerplane ofthe tire 5. The composite cord 20 can also be used in the belt structure15 where it is desirable to produce the tire 5 by the flat band methodwherein the components of the tire are laid up on a cylindrical tirebuilding drum.

Thus, it can be appreciated that all of the reinforcement cords can beof the composite type, as described, in cases where the tire is moldedwith a reduced diameter for use as a spare tire. Also the individualcomponents of the tire 5, such as the belt structure 15 or carcassoverlay 18, can be formed of such composite cords 20 to allowunrestricted expansion of the tire at least to the point where thesubstantially inextensible yarns 22,23 become twisted together andtensioned.

It will be understood that with the cords 20 of the present invention arelatively high elongation may be easily obtained. It takes relativelylittle effort or work to stretch out the cords 20 to their full extent,because the core 21 offers little or no resistance to the stretching outof the individual yarns 22,23 of each cord after being subjected totemperatures normally used for vulcanization.

Thus, there has been described a tire 5 that is reinforced withcomposite cords 20 composed of substantially inextensible yarns 22,23which are spirally wrapped around a core 21 of polyethylene orpolypropylene that loses its tensile strength during the molding andvulcanization process, thereby allowing elongation of the compositecores as the individual yarns stretch out.

It will be clear that the cord of the present invention may also be usedin articles other than tires without departing from the scope of thepresent invention.

What is claimed is:
 1. A composite cord for reinforcing an elastomericarticle, comprising at least one high tenacity substantiallyinextensible yarn spirally wrapped around a core, said core including atleast one filament having at room temperature an elastic limit of atleast b 2.5 kilograms and a modulus of elasticity greater than 25 gramsper denier providing a barrier to elongation to resist and recover fromtension forces on the cord during the processing of said cord. andelastomeric article until said core is subjected to temperaturesnormally used during vulcanization of rubber and thereafter said corehaving a reduction in tenacity after being subjected to temperaturesnormally used during vulcanization of rubber to permit stretching out ofsaid yarn when said core is cooled to allow for elongation of theelastomeric article and then reinforcing of the article in the elongatedcondition when said yarn is stretched out.
 2. The cord of claim 1wherein said temperatures normally used during vulcanization are in therange of 120° C. to 150° C.
 3. The cord of claim 1 wherein the core iscomposed of a polymer of the group consisting of polyolefines.
 4. Thecord of claim 3 wherein said polyolefines include polyethylene andpolypropylene.
 5. The cord of claim 1 wherein the core is amonofilament.
 6. The cord of claim 1 wherein the core has an elasticlimit of at least 1.5 grams per denier before being subjected to saidtemperatures normally used during vulcanization.
 7. The cord of claim 2wherein the core has said reduction in tenacity at a temperature in therange of from 130° C. to 140° C.
 8. The cord of claim 1 wherein saidyarn is composed of at least one twisted strand which is composed offilaments of reinforcing material.
 9. The cord of claim 8 wherein saidyarn is composed of said strand cabled together with other strands. 10.The cord of claim 8 wherein said strand has a denier in the range offrom 800 to
 1500. 11. The cord of claim 1 wherein a second substantiallyinextensible yarn of high tenacity is spirally wrapped around said corein opposed relation to said first yarn so that after said reduction intenacity of said core and elongation of said cord said first and secondyarns will twist together.
 12. The cord of claim 1 wherein said yarn iswound around the core in the range of from 3 to 7 turns per centimeterand said cord is elongatable between 50 percent and 100 percent.